1. Field of the Invention
The present invention relates to an electronic part and a method of manufacturing same, and more particularly to a monolithic ceramic electronic device and a method of manufacturing same.
The present invention can be adapted to a variety of electronic devices, e.g. a monolithic capacitor, a monolithic piezoelectric device or a multilayered ceramic substrate, and a method of manufacturing such devices.
2. Related Background Art
Methods are known of integrally firing metal elements and ceramic elements for manufacturing a monolithic ceramic electronic device, such as a monolithic capacitor, having internal electrodes.
For example, a pattern made of conductive paste is printed on a ceramic green sheet to form internal electrodes. Then, a plurality of the ceramic green sheets each having the internal electrodes are laminated, and then an appropriate number of ceramic green sheets having no internal electrodes are laminated on both sides of the stack of green sheets to form a monolithic ceramic structure. Alternatively, predetermined patterns of ceramic paste and conductive paste may be sequentially printed to form a monolithic ceramic structure.
Then, the monolithic ceramic structure is pressed in a direction of its thickness so that the ceramic layers are brought into tight contact with each other. Further, the monolithic ceramic structure is fired to obtain a sintered structure. Appropriate external electrodes are formed on the outer surface of the sintered structure to produce a monolithic ceramic electronic part.
In recent years it has been required to reduce the sizes of electronic parts, as well as the sizes and thicknesses of monolithic ceramic electronic devices. When the monolithic ceramic electronic device is intended to be reduced in size and thickness, the ceramic layers, each of which is held between the internal electrodes, are required to be made thinner. Accordingly, thinner green sheets are required when the monolithic ceramic structure is manufactured.
However, it is difficult to handle excessively thin ceramic green sheets. Moreover, the portions including the internal electrodes, which overlap one another, are thicker than the portions not including internal electrode in a monolithic ceramic structure. As a result, steps have been inevitably formed between these portions.
In particular, since the steps are formed when the monolithic ceramic structure is pressed in the direction of its thickness prior to the firing process, the pressure is mainly applied onto the portions where the internal electrodes overlap one another. Accordingly, the pressure is insufficient in the other portions. As a result, there is a possibility that a layer-separating phenomenon called delamination may occur. Also, there is a possibility that solvent in the ceramic green sheet may cause the internal electrodes to be swelled. If that occurs, the internal electrodes cannot accurately be formed into desired shapes.
Therefore, it has been extremely difficult to reduce the thickness of a ceramic green sheet to about 6 .mu.m or thinner.
Such problems may arise in making a monolithic ceramic structure in which ceramic paste and conductive paste are alternately laminated.
To overcome these problems, a method has been suggested in which a metal film, formed by a thin film forming method, is used as the internal electrodes, the method being exemplified by the following first to third methods.
The first method, shown in FIG. 1, begins with the step of forming a metal film over the entire surface of a supporting member 1 by using a thin film forming method, such as a sputtering method. Then, a resist layer having openings corresponding to the shapes of the electrodes is formed on the metal film, and then the metal film is patterned by photolithography. Thus, a metal film 2 is formed as shown in FIG. 1. Then, a ceramic green sheet 3 is formed on the metal film 2. By repeating the steps of forming the metal film 2 and forming the ceramic green sheet 3, a monolithic structure 4 is formed.
In the second method, which is disclosed in JP-A-64-42809, a ceramic green sheet is formed on a first film made of synthetic resin; and a metal film is formed on a second supporting film by a thin film forming method. Then, the metal film supported by the second supporting film is transferred to the ceramic green sheet on the first supporting film so that the green sheet has the metal film thereon. A monolithic ceramic structure is obtained by laminating a plurality of such green sheets.
The third method begins with the step of forming a metal film on the entire surface of a supporting film by a thin film forming method. Then, the metal film is patterned by a photolithography method. Next, a ceramic green sheet is formed on the supporting film having the patterned metal film thereon so that the green sheet is combined with the metal film. Then, the green sheet supported by the supporting film is transferred onto a substrate by using a thermal transfer method so that a monolithic ceramic structure is obtained.
The first to third methods, in each of which metal films formed by a thin film forming method are used as the internal electrodes, are able to make the internal electrodes thinner as compared with the method of forming the internal electrodes by using conductive paste.
However, past attempts to make each layer thinner have resulted in an increase in the number of laminated layers, thus causing the internal electrodes to be thickened in comparison with the thickness of the ceramic layer held between the internal electrodes. As a result, as shown in FIG. 2, the first method results in a difference in thickness between a portion 6, in which only the ceramic green sheets are laminated, and a portion 7, in which internal electrodes 8 overlap one another. Therefore, when the laminated structure is pressed in the direction of its thickness, the pressure is primarily applied only to the portion in which the internal electrodes 8 overlap one another. Thus, the strength of adhesion between the ceramic layers may be reduced in the region 6 in which the internal electrodes do not overlap. As a result, delamination may easily take place when the laminated structure is sintered.
Moreover, the first method requires further process steps after the metal film has been formed on the supporting member, such as a step of forming patterns of a resist layer, a step of etching the pattern and a step of stripping the resist layer off.
The second method also encounters the problem of thickening of the portion in which the internal electrodes overlap one another, as compared with the region having no internal electrodes. Therefore, the second method also may suffer from the delamination problem. Further, in the case where the ceramic green sheets are made extremely thin, it would be difficult to handle such green sheets. Moreover, it would be difficult to position the metal film on a green sheet with high accuracy because a transferring process is involved.
And, in the transferring process, the ceramic green sheet must be brought into contact with the second supporting film in order to make the portion having no metal film. The second supporting film must be stripped from the green sheet having the metal film after the transferring process. Therefore, both the metal film and the ceramic green sheet must be able to be easily stripped from the second supporting film. However, this requirement can not easily be satisfied. Thus, there is a possibility that a portion of a ceramic green sheet may be destroyed when stripping off the second supporting film.
Since the third method has the steps of forming the metal film by the thin film forming method and then performing the patterning process by the photolithography method, this manufacturing process tends to become complicated. Moreover, making the structure in which the metal film and the ceramic green sheet are in contact with the supporting film requires both the metal film and the ceramic green sheet to be easily stripped from the supporting film when transference from the supporting film is performed. However, this requirement cannot easily be satisfied.